This invention relates to color television demodulator systems and more particularly to a chroma demodulator section of a television receiver for receiving Secam broadcast signals wherein a tunable discriminator is provided for tuning the demodulator to the Secam reference frequencies.
In Secam color television receiver systems the chrominance signals R-Y and B-Y are transmitted sequentially line by line by frequency modulation of an RF subcarriers as is known. To allow correct identification of these signals in the receiver the R-Y subcarrier has a frequency of 4.40625 megaHertz and the B-Y subcarrier a frequency of 4.250 megaHertz. These two reference frequencies are commonly referred to as f.sub.o R and f.sub.o B respectively. To permit correct hues to be regenerated in the television receiver the frequency discriminator section of the chroma demodulator must provide a zero (or a precisely known) output at both the frequency f.sub.o R of the R-Y line and at the frequency f.sub.o B of the B-Y line.
Contemporary Secam television receivers usually meet this requirement by using two frequency discriminators, e.g., Foster-Seeley discriminators, each tuned respectively to the center frequencies f.sub.o R and f.sub.o B. However, to maintain visibly neutral gray tones, the tuning accuracies must be typically .+-.5 KHz on each of the two discriminators. This accuracy must be maintained during the life of the receiver and over the full ambient temperature range in which the television receivers exist. Such performance is not attainable with components and present day manufacturing techniques.
To overcome the above problems, various attempts have been made to use the burst of the f.sub.o R and f.sub.o B frequencies, which are transmitted at the beginning of the R-Y and B-Y lines respectively, as a precise frequency reference. A number of technical difficulties however have made this impractical.
Another prior art system for demodulating the Secam chrominance signals uses a single frequency discriminator to which are fed the R-Y and B-Y lines in sequence, yielding demodulated outputs of the R-Y and B-Y information in sequence. These signals are then passed via direct and delayed paths to a commutator operating at half the line frequency such that at one output of the commutator there appears the R-Y signal and at the other output appears the B-Y signal. A problem exists in these system due to the fact that the delay lines, which are almost universally employed today, are glass delay lines and operate only at RF frequency. Therefore, the signal to be delayed must first be amplitude modulated onto a suitable subcarrier then demodulated after passage through the delay line. This subcarrier frequency must be precise and typically is provided by a reference oscillator incorporating a crystal.
It is an aspect of the present invention to provide an improved demodulator system suitable for use in Secam color television receivers.
Another aspect of the present invention is to provide a demodulator for a Secam color television receiver including a single tunable discriminator for setting the demodulating system to the sequential reference frequencies.